A standard magnetic bubble memory (MBM) system includes, in addition to the MBM device itself, a number of support circuits. These support circuits produce current pulses and timing for control of read and write operations, generate clocks for circulation of data around loops, convert the output signals to usable logic levels, and map out redundant loops from the device. Typically, the support circuitry includes a bubble memory controller (BMC), a function timing generator, a function driver, coil drivers and a sense amplifier used to detect the existence or absence of a bubble at the detector element.
The bubble memory controller (BMC) is typically used for interfacing the MBM system with a microcomputer. The BMC may include a controller integrated circuit IC, which typically includes a decoder, control registers and counters, a comparator, a FIFO (first-in, first-out buffer), a clock distributor and a microsequencer used to interpret a command and to initiate a sequence of control signals to execute a read or write data transfer operation. A function timing generator also forms part of the controller, and interfaces the controller with the coil drivers (which interface clock signals to the MBM), the function driver (which interfaces the function drive outputs of the function timing generator to the MBM), and the sense amplifier of the memory module.
The function timing generator generates coil drive signals, senses power failures, controls power down, and develops various of the precisely timed function signals needed during the read and write cycles to synchronize the operation of the several devices included in the bubble memory module. Thus, pulses produced by the function timing generator are converted to higher level voltages which are integrated by the driver coils to produce the desired triangularly shaped currents needed during reading and writing of the MBM and for moving the bubbles through the various tracks and loops of the device.
Such typical magnetic bubble memory devices are described in Handbook of Semiconductor and Bubble Memories, Walter A. Triebel, et al., Prentice Hall, Inc., Englewood Cliffs, N.J. 07632, 1982.
Since all operations which move, create, or destroy bubble data on the surface of the bubble substrate of a bubble memory are magnetically induced, such bubble memories are extremely reliable with respect to radiation and are not susceptible to radiation damage in view of the insensitivity of the magnetic domains to effects of radiation. That is, for radiation to affect the stored bubble data it must either generate a magnetic field or modify the garnet material sufficiently to spoil its magnetic properties. Neither of these conditions exist for practical nuclear or single event particle conditions. Since no electric field or electric amplification exists in the device, no particle radiation (or ionizing radiation) can generate the current necessary to modify the bubble patterns. Moreover, the garnet medium is similarly undamaged by total ionizing dose and by neutron fluence, which would damage any semiconductor device.
However, the various external support circuits of a bubble memory, which produce magnetic fields for controlling the bubbles stored in the device, are sensitive to ionizing radiation, whether viewed as an ionizing radiation transient or as a single particle event. Thus, for bubble memories, which are moving data memories rather than moving media memories, either degradation or upset caused by the effects of radiation on the support circuitry may be catastrophic, even for single upsets of short duration.
The prior art has generally attempted to solve the problem of radiation sensitivity by attempting to insulate circuits from, rather than permitting circuits to operate in the presence of, such ionizing radiation. Thus, thinner and purer gate oxides have been used to reduce radiation sensitivity of NMOS IC logic used in bubble chip sets. CMOS circuit technology has overtaken NMOS logic, and is inherently harder (more insensitive to radiation damage) than is NMOS circuitry.
However, whether CMOS or NMOS circuitry is involved, both total radiation dosage and dose rate must be considered in hardening bubble memories to the effects of radiation.
In one prior art attempt to overcome the above noted dose rate problem, redundant bubble devices have been used. Therein, all critical data is stored in two sets of bubble devices, with one set left unpowered at any given time. If data in one set is scrambled, data from the second set is used. One illustration of memory redundancy to provide radiation protection is disclosed in Sabo, et al., U.S. Pat. No. 4,413,327. Therein, memories are interrupted during an event and a safe memory provided to an update circuit after conclusion of the event.
However, such approaches suffer from three distinct disadvantages. The added storage capability increases overhead circuitry. The increased overhead may approach 100% if all data are considered critical Moreover, because it is necessary to re-write completely the scrambled bubble device, including reconstruction of the bootloops thereof, recovery from radiation exposure is slow when redundant bubble memories are used. Finally, until complete data reconstruction is performed, the system is vulnerable to a second nuclear event.
In another approach to hardening of bubble memories, the prior art has attempted to assure that the logic states controlling the bubble device are never upset. However, to implement such an approach several thousand flip-flops are required to store the bubble memory state during a nuclear event. Thus, the prior art approach to avoidance of upset in the logic states of a MBM, and the loss of data associated therewith, is quite costly in hardware requirements.
In order to provide the desired degree of nuclear hardness, however, the inventor hereof has concluded that it is necessary to design a bubble chip set which will assure that minimum acceptable parameters are met for a minimal number of critical drive functions of the bubble memory. Further, in order to limit the scope of the problem to be solved, and thus to limit the costs and overhead associated with the solution, it is necessary to provide continuous certainty and reliability for only those functions critical to the storage of data. That is, various field generating functions treated as critical may be protected, while various other functions associated with the bubble memory may be treated as noncritical and are thus only required to work properly before and after a nuclear transient, but not during such an event.
The prior art has failed to recognize this concept, and has accordingly provided expensive redundancies to various circuits. Some of the prior art approaches to hardening a storage to the effects of radiation are described below.
In U.S. Pat. No. 4,031,374 of Groudan, et al., assigned to the assignee hereof, there is disclosed circumvent circuitry for limiting all currents in a memory access network of a plated wire or magnetic core memory during a radiation event. The disclosed arrangement thus provides correction of single word errors in a memory environment wherein only the word being read or written may be lost. However, for a MBM environment loss of a single word in a magnetic bubble memory is tolerable. On the other hand, if a critical function is disrupted, the entire memory may become useless. Accordingly, the '374 patent does not address the problems described herein and avoids only the loss of a single word. In the present invention, however, the entire memory is protected by completion of a rotation cycle, for example, or by protection of other critical functions. Moreover, in the '374 patent, circumvent circuitry currents are limited in all selection lines to less than one-half the select current. In the present invention, in order to complete the rotation cycle the drive currents are maintained at, or close to, the appropriate full drive levels.
In U.S. Pat. No. 4,464,752 of Schroeder, et al., similarly assigned to the assignee hereof, there is described a 5 ms annealing period, during which a memory is clamped, and a recovery period to reconstruct data so that an individual word may be reconstructed, in a writing or reading operation. Specific circuitry is required to assure complete hardness, and the clamp period is extended to variable time. Flags are used to indicate that the memory was cycling during a radiation event, thus raising the possibility that data may have been lost. This prior disclosure, however, does not relate to a bubble memory and thus does not rely on the inherent radiation insensitivity of the storage itself. By providing multiple word error corrections in random time periods, correction is provided for data altered during a multiple nuclear event sequence However, no suggestion may be derived from the disclosure that only a limited number of function generating circuits be protected, such as circuits for assuring that an access cycle be completed and for inhibiting only subsequent SWAP cycles. Nor may a suggestion be inferred therefrom to harden only the drive circuits or to harden such circuits by a specific approach, as described herein.
Thus, by attempting to attain a goal of complete reliability of both volatile and nonvolatile data, and by seeking to provide nuclear hardness for each of the memory devices as well as the circuits associated therewith, the prior art has overlooked a simpler, and less expensive, solution to radiation problems for bubble memories which are inherently insensitive to radiation. The prior art has ignored the fact that no alteration of nonvolatile bubble data will occur due to a nuclear event, provided that a minimal number of criteria are met. The art has, more specifically, overlooked the possibility of providing different degrees of protection to different portions of the support circuitry provided for a bubble memory in order to safeguard completely the data stored in the memory.